Ceramic matrix composite (CMC) materials are known for use in high temperature environments such as along the hot combustion gas flow path of a gas turbine engine. CMC materials include ceramic reinforcing fibers disposed in a ceramic matrix material. FIG. 1 provides a partial cross-sectional view of a prior art article 10 formed of a plurality of plies 12, 14, 16 of a ceramic matrix composite material. Each of the plies 12, 14, 16 contains a 2-D weave of reinforcing fibers 18 encased within a matrix material 20. While the design of such an article assumes an intimate contact sintered bond between adjacent plies 12, 14, 16, what are seen in FIG. 1 are delaminations 22, 24 formed between adjacent plies in a non-planar region of the article 10. These delaminations 22, 24 are the result of the anisotropic shrinkage of the CMC material that occurs during the processing of the article 10. Anisotropic shrinkage results from the shrinkage of the matrix material 20 being greater than the shrinkage of the fibers 18 during the drying/curing/sintering of the material. Thus, a ply of the CMC material will tend to shrink more in its thru-thickness direction than in the in-plane direction of the fibers, tending to form delamination between adjacent plies. This is especially true in a non-planar region 26 where such anisotropic shrinkage drives a degree of relative movement between adjoining plies. When the in-plane adhesion between the plies in the planar regions 28 is stronger than the interlaminar adhesion between the plies in the non-planar region 26, at least a portion of the relative movement between adjoining plies may be accommodated by the generation of the delaminations 22, 24.
Known methods of manufacturing ceramic articles include hot pressing and hot isostatic pressing (HIP). These methods are capable of producing the very high pressures required to achieve fully dense (approaching zero porosity) ceramic materials. The present inventors are aware of such hot pressing processes being used at pressures ranging from as low as 750 psig to well above 10,000 psig. These methods are not used when manufacturing known oxide-oxide ceramic matrix composite materials, since a relatively high degree of porosity (10-30%) is generally required to provide a desired degree of fracture toughness in the fully sintered material. Accordingly, prior art multi-layer oxide-oxide CMC's, such as those including 2D alumino-silicate fibers within an alumina, alumina/mullite or mullite matrix, are generally sintered in an unsupported condition after being dried to a green state. In one example, a plurality of plies of CMC material containing alumino-silicate fibers in an alumina-containing matrix may be laid up against a mold using a vacuum bagging process, then dried to a green state in an autoclave at approximately 80 psig, then sintered in a furnace in an unsupported configuration. It is during the sintering step that delaminations 22, 24 often occur between the plies 12, 14, 16, with complex-shaped articles 10 including non-planar regions 26 being the most susceptible to the formation of such delaminations 22, 24. Even when significantly large delaminations are not formed, the resulting structure will exhibit significantly lower interlaminar strength than in-plane strength, with values for the example prior art materials typically being 4 MPa minimum (6 MPa average) interlaminar tensile strength and 140 MPa minimum in-plane strength. Higher strength materials are needed for certain applications and materials without delaminations are desired for all applications.